1. Field of the Invention
The present invention relates generally to a fire heat sensor, and more particularly to a fire heat sensor that performs differential heat sensing, i.e., a fire heat sensor that detects a fire by judging the rate of a rise in temperature by a pair of temperature detecting elements and a heat conduction structure thereof.
2. Description of the Related Art
There is a conventional fire heat sensor that performs differential heat sensing. The differential fire heat sensor detects a fire by judging the rate of a rise in temperature caused by the fire. As such a differential fire heat sensor, there are a thermocouple type heat sensor, and a heat sensor employing two thermistors. In addition, there is a temperature sensor employing a fine machining technique for purposes of detecting a sharp change in temperature. These sensors are used to detect a sharp rise in temperature, based on a difference in temperature between two points. To cause the temperature difference to occur, one of the two points has a fast response to heat and the other point has a slow response to heat.
FIG. 13 shows a conventional fire heat sensor with two thermistors as heat sensing elements (see Japanese Laid-Open Patent Publication No. HEI 1-297795). In this type of fire heat sensor, one (thermistor 101) of the two thermistors has a fast response to heat because it is exposed to hot airflow, and serves as a high-temperature detecting portion. The other thermistor 102 has a slow response to heat because it is housed within a cover, and serves as a low-temperature detecting portion.
When the fire heat sensor is exposed to hot airflow, the temperature detected by the first thermistor 101 changes sharply because the heat response is fast. On the other hand, the temperature detected by the second thermistor 102 changes slowly because the heat response is slow. Therefore, a temperature difference signal of a sufficient magnitude is obtained. When it exceeds a predetermined threshold value, the heat sensor can judge the occurrence of a fire.
As described above, in the differential type fire heat sensor, a difference in temperature is detected by two temperature detecting elements having a fast response to heat and a slow response to heat. Because of this, the level of a temperature difference due to a sharp change in temperature caused by a fire cannot be easily discriminated from the level of a temperature difference due to a gradual temperature change. To discriminate between the two levels, signal processing is required.
FIG. 14 shows the principles of a conventional differential fire heat sensor. The temperature detecting element 201 of a high-temperature detecting portion is situated at a position where hot airflow is directly exposed, while the temperature detecting element 202 of a low-temperature detection portion is situated at another position where the hot airflow is screened by a guard member 203.
FIG. 15 shows how a high temperature Th detected by the high-temperature detecting element 201, a low temperature Tc detected by the low-temperature detecting element 202, and a temperature difference ΔT, are changed when the ambient temperature Ta in FIG. 14 rises sharply. In this case, the high temperature Th rises sharply, and the low temperature Tc rises slowly. As a result, a great temperature difference ΔT is obtained.
FIG. 16 shows how the above-described high temperature Th, low temperature Tc, and temperature difference ΔT are changed when the ambient temperature Ta in FIG. 14 rises slowly. In this case, the high temperature Th rises along with the ambient temperature Ta, and the low temperature Tc rises slowly. Because of this, as with the case of the sharp temperature change in FIG. 15, a great temperature difference ΔT is obtained.
However, in the case of the differential heat sensing in which the occurrence of a fire is judged when the temperature difference ΔT exceeds a predetermined level TH, the temperature difference ΔT exceeds the predetermined level TH even when the ambient temperature Ta changes slowly. Because of this, to discriminate a sharp temperature rise from a slow temperature rise, the case of the sharp temperature rise requires a temperature characteristic F (ΔT), as shown in FIG. 15. The case of the slow temperature rise requires a temperature characteristic F (ΔT), as shown in FIG. 16. Because of this, the differential heat sensing circuit becomes complicated.
Furthermore, the high-temperature detecting element 201 and the low-temperature detecting element 202 are situated at asymmetrical positions with respect to the horizontal direction, so the heat response of the low-temperature detecting element 202 varies with the direction of hot airflow. Because of this, the differential heat sensing, based on a difference in temperature, greatly depends on the direction of hot airflow.